The studies proposed in this competitive renewal are directed at under- standing neurophysiological bases of perceptual performance in auditory detection and discrimination tasks in cats and humans. Two lines of work will be pursued initially. The first consists of acute electrophysiological studies that employ 2AFC and yes-no methods to explore the ability of single auditory nerve fibers to detect and discriminate acoustic signals across specific dimensions. The goals of the work is to examine the degree to which temporal-and rate-based coding schemes account for perceptual performance. Specific tasks to be studied include frequency, intensity and temporal gap discrimination, forward and simultaneous masking, and frequency and amplitude modulation detection. To gain a more complete picture of neural processes involved in perceptual decision-making, experiments will be performed in normal animals, animals with sensorineural hearing loss, and animals with cochlear implants. The second line of work is the development of an excitation pattern / neural population response model that incorporates all relevant sources of response variability and generates, on a fiber-by-fiber basis, accurate estimates of the auditory nerve population response to arbitrary signals. The model will be used to examine the extent to which neural population responses anticipate perceptual performance on the tasks listed above. Later, the model will be modified and extended to consider population-based coding schemes in condition of hearing impairment. Midway through the award period an animal psychophysics facility will be established, and detection and discrimination data will be obtained from behaving cats. The goals of this work are to fill in gaps in knowledge of the perceptual capabilities of animals and to generate psychophysical data whose neural substrates can be examined electrophysiologically and modeled. Animals trained to perform perceptual tasks will become subjects for electrophysiological experiments. Their cochleas will also be processed histologically to determine spatial distributions of spiral ganglion cell counts, and the anatomic data will be incorporated into population response models tailored to each animal. This approach allows perceptual findings to be directly compared with single-cell responses and the performance of anatomically accurate population models on identical tasks.